1. Field of the Invention
The present invention relates to a method and a device that can be used especially to demodulate the signals received in an interrogator-responder type of communications system.
The invention can be applied for example for the demodulation of IFF (Identification Friend or Foe) signals using different waveforms, such as those of MSK (Minimum Shift Keying), DPSK (Differential Phase-Shift Keying), FSK (Frequency Shift Keying) etc.
2. Description of the Prior Art
In interrogation-response type communications systems, for example of the IFF type, the devices are generally dedicated to a particular encoding technique. This implies the development of a device with applications-specific components.
In the IFF field, for example, different waveforms may be used. The modulations used classically are listed in the following table.
Type d'interrogation/responseModulationInterrogations/ResponsesPulsesModes 1, 2, 3/A, B, C, D, testand Mode 4Mode S InterrogationsPulses + DPSK (Differential Phase-Shift Keying)Mode S ResponsesPPM (Pulse Position Modulation) typepulsesMode 5 Interrogations/Pulses + MSK (Minimum ShiftResponsesKeying)
At present, each type of modulation has a decoding technique associated with it.
FIG. 1 shows an exemplary demodulation chain used for DPSK modulation. This chain comprises, for example, two reception antennas, one high antenna 1h and one low antenna 1b, referenced as a function of their position in the receiver, a ‘Dual Transponder/Interrogator Receiver’ (DTIR) unit and a signal processor unit Ts. To decode DPSK-modulated signals, a delay line or a Costa loop, for example, is used in the reception chain.
The analog signals exchanged in the communications system are received on the high antenna 1h and on the low antenna 1b of the receiver of the communications system. The signal Sh received by the high antenna and the signal Sb received by the low antenna are processed identically. For reasons of simplification of the description, only the sequencing of the processing steps carried out on the signal Sh shall be explained in detail. To obtain a description of the processing of the signal Sb, it is enough to replace the index h by the index b corresponding to the low antenna.
The analog signal Sh is mixed, in a mixer 3h, at a mixing frequency Fm whose value is equal, for example, to 960 MHz or to 1090 MHz. At the end of this step, a signal Sh(Fl) is generated at an intermediate frequency Fl of about 60 or 70 MHz. The mixing frequency value Fm generated by the local oscillator 4, substantially corresponds, for example, to the value of the frequency of response of the system. The signal Sh(Fl) thus obtained is then transmitted to a device 5h whose function especially is to separate the signal into a video signal Svh and a limited intermediate frequency signal Sdh containing the data. This signal Sdh is independent of the power received at the antenna. The video signal Svh is transmitted directly to a processing card 10 (along the channel V in the figure). A switch 6 is used to select the signal to be processed. It is controlled, for example, by the video signal Svh or Svb. The data signal Sdh(Fl) is sent to the switch 6, then to a delay line 7 adapted to producing two signals, offset with respect to each other in time. These two signals, Sdh(t), Sdh(t+τ) respectively have a phase shift Δφ. They are sent in a mixer 8 which determines the value of the phase shift Δφ. This phase shift is then transmitted to a comparator 9 set to a decision threshold D, which thus generates a binary signal formed by zeros and ones. This binary signal Sbh is transmitted to the processing card 10 which thus receives the video signal Svh and combines these two signals to determine the signal Sh received by the high antenna 1h. 
These processing steps are also applied to the demodulation of the signal Sb received at the low antenna.
FIG. 2 shows an exemplary prior art reception chain for the MSK Mode 5.
The first Dual Transponder/Interrogator Receiver (DTIR) part is identical to that of FIG. 1 and shall therefore not be described for FIG. 2.
At output of the Dual Transponder/Interrogator Receiver (DTIR) device, the intermediate frequency signals Sdh(Fl) and Sdb(Fl) and the corresponding video signals Svh, Svb are processed as follows:                The video signal Svh, Svb is directly transmitted to the processing card 10 (along the channel V)        The intermediate frequency signal Sdh(Fl) and Sdb(Fl) is transmitted to an I, Q type demodulator referenced 12h, 12b, which works at a frequency delivered by a local oscillator 11. It may also have its own integrated clock. At output of the demodulator, the signal is filtered and amplified by an appropriate device 13h, 13b. The signals I and Q thus obtained are transmitted to the processing card 10, which also receives the video signal Svh, Svb and combines these two signals to obtain the information corresponding to the decoded signal.        